Oil well pipe for a casing

ABSTRACT

An oil well pipe for a casing, comprising an oil well body, a sliding layer formed on an outside surface of the oil well body and a protective layer formed on an outside surface of the sliding layer, the sliding layer contains an ethylene-vinyl acetate copolymer having 5 to 60 weight % vinyl acetate. The sliding layer may also contain a maleic acid modified polyethylene or the ethylene-vinyl acetate copolymer may be a maleic acid modified ethylene-vinyl acetate copolymer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a oil well pipe for casing, which is not destroyed by ground subsidence.

2. Description of the Related Arts

An oil well for extracting crude oil is formed by a tubing, which is an oil well pipe for drawing up crude oil from the oil reservoir to the ground, and a casing, which is an oil well pipe for protecting the tubing. Since most oil wells are bored substantially in the vertical direction, a downward pulling frictional force acts on the oil well pipe for casing (hereinafter called oil well pipe) due to ground subsidence caused by the extraction of crude oil etc.

In the conventional oil well, however, the aforementioned frictional force acting on the oil well pipe is not always high, so that the possibility that the oil well pipe is destroyed by the frictional force is not always high. Therefore, the measures for reducing the frictional force have not been taken.

Recently, the problem of oil well pipes destroyed by ground subsidence has become serious as the oil wells become deep. The oil well pipes buried in a great depth are subjected to a high underground pressure, so that the acting frictional force is high, and the destruction of oil well pipes cannot be neglected. For this reason, the measures for reducing the frictional force acting between the ground and the oil well pipe have been taken, and the development of an oil well pipe which is not destroyed by ground subsidence has been desired.

SUMMARY OF THE INVENTION

It is an object of the present invention provide an oil well pipe which is less prone to be destroyed by ground subsidence by reducing the frictional force acting on the oil well pipe and a method for making the same.

To achieve the object, the present invention provide an oil well pipe comprising:

an oil well pipe body; and

a sliding layer formed on the outside of said oil well pipe body.

Moreover, the present invention provides a method for making a oil well pipe comprising the steps of:

preparing an oil well pipe body; and

forming a sliding layer on the outside of said oil well pipe body.

The sliding layer can be made from a material having a stiffness S(t) after t seconds of 10⁻⁵ to 10³ N/m² after one year or more has elapsed at a working temperature of the oil well pipe. The stiffness S(t) is expressed by the following equation using a deformation amount d(t) mm after t seconds, a frictional force τ N/m² and a thickness h mm of the sliding layer:

    S(t)=3h·τ/d(t).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of an oil well pipe or the present invention;

FIG. 2 shows a schematic view illustrating a method for making an oil well pipe of the present invention; and

FIG. 3 shows a schematic view illustrating the viscous behavior of a sliding layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The oil well pipe in accordance with the present invention has an oil well pipe body and a sliding layer formed on the outside of the oil well pipe body.

If a frictional force due to ground subsidence acts on an oil well pipe buried vertically under the ground, the sliding layer formed on the outside of the oil well pipe and having viscosity is pulled down by a frictional force and is deformed, thereby reducing the frictional force acting on the oil well pipe body.

Specifically, as shown in the schematic view of FIG. 3, when a constant shearing stress (frictional force) τ acts on the upper surface of a sliding layer 21 showing viscous behavior and having a thickness h, sliding deformation (viscous behavior) as indicated by the broken line in the figure occurs after t seconds.

The relationship between the deformation amount d(t) after t seconds and the frictional force τ can be expressed by Equation (1) using the stiffness S(t) and the thickness h of the sliding layer.

    τ=S(t)·d(t)/3h                                (1)

It can be seen from Eq.(1) that the use of a material having a low stiffness S(t) for the sliding layer decreases the frictional force τ.

The inventors of the present invention conducted model experiments and found that the frictional force τ should be 2×10³ N/m² or lower in order for the oil well pipe to be less prone to be destroyed by ground subsidence. Further, we found that the destruction of oil well pipe can be prevented almost completely by reducing the frictional force τ to 2×10² N/m² or lower.

From these results, we determined a value of 10³ N/m² as the stiffness S(t) of the sliding layer which does not cause destruction of oil well pipe assuming that the thickness h of the sliding layer is 2 mm, the deformation amount d(t) or the sliding layer after one year is 20 mm, and a frictional force of 2×10³ N/m² continues to act.

It was found that if the stiffness S(t) after one year has elapsed is 10³ N/m² or lower, the oil well pipe is less prone to be destroyed, and if the stiffness S(t) is 10² N/m² or lower, which is the case corresponding to a frictional force of 2×10² N/m² or lower, the destruction of oil well pipe does not occur substantially.

In other words, a material having a stiffness S(t) after one year exceeding 10³ N/m² cannot reduce the frictional force because the deformability is insufficient, having a high possibility of destruction of oil well pipe.

It was also found that if the stiffness S(t) after one year is lower than 10⁻⁵ N/m², the sliding layer is softened and melted.

The stiffness S(t) of the material in accordance with the present invention decreases gradually before one year has elapses. However, after one year has elapsed, the stiffness S(t) scarcely change, being substantially equal to the value at the time when one year has elapsed. Therefore, the range of the stiffness S(t) required is the same after one year has elapsed.

As described above, the oil well pipe in accordance with the present invention has a reduced frictional force between the subsiding ground and the oil well pipe body, preventing the destruction of the oil well pipe, so that a material for the sliding layer is selected in accordance with the temperature at which the oil well pipe is used.

That is to say, the material used for the sliding layer must naturally show the intended viscous behavior in the intended temperature range.

Therefore, when the oil well pipe is used in the temperature range of 100° to 130° C., which now poses a problem of destruction of oil well pipe, a material having a stiffness S(t) in the range of 10⁻⁵ to 10³ N/m² in the temperature range of 100° to 130° C., preferably a material in the range of 10⁻⁵ to 10² N/m², should be selected as the material for the sliding layer to prevent the destruction of oil well pipe.

Needless to say, it is preferable that this condition should be met even in the temperature range other than the above range. Preferably, a material which has a stiffness S(t) in the above range in a temperature range as wide as possible is selected.

At present, the temperature which oil well pipes are subjected to is 130° C. or lower. As, the result of various investigations on materials meeting the above conditions, it was found that several materials can be used as the sliding layer.

First, asphalt can provide a material having a stiffness S(t) after one year in the range of 10⁻⁵ to 10³ N/m² in the temperature range of 40° to 100° C. by mixing straight asphalt and heated asphalt in the predetermined proportions. The asphalt thus obtained can contain additives such as resin, fat and oil, rubber, pigment, and filler.

Polyethylene can also be used. The softening point of polyethylene can be changed by changing its density. For example, when the oil well pipe is used in the temperature range of 100° to 130° C., polyethylene having a density of 0.930 g/cm³ or lower must be selected to meet the condition of the above range of stiffness S(t) in this temperature range.

Polyethylene having a density exceeding 0.930 g/cm³ has a stiffness S(t) after one year of 10³ N/m² or higher at 100° C., so that it does not soften sufficiently, being incapable of performing the function as the sliding layer. Contrarily, polyethylene having a density of 0.930 g/cm³ or lower has a softening temperature lower than 100° C. so that it can perform the function as the sliding layer because its stiffness S(t) after one year at 100° C. is 10³ N/m² or lower.

For polyethylene having a low density (0.930 g/cm³ or lower), the lower limit of density need not be specified because the condition that the stiffness S(t) after one year at 130° C. is 10⁻⁵ N/m² or higher is met.

Maleic acid modified polyethylene can also be used. The softening point of maleic acid modified polyethylene can be changed by changing its density. For example, when the oil well pipe is used in the temperature range of 100° to 130° C. maleic acid modified polyethylene having a density of 0.930 g/cm³ or lower must be selected to meet the condition of the above range of stiffness S(t) in this temperature range.

Maleic acid modified polyethylene having a density exceeding 0.930 g/cm² has a stiffness S(t) after one year of 10³ N/m² or higher at 100° C. so that it does not soften sufficiently, being incapable of performing the function as the sliding layer. Contrarily, maleic acid modified polyethylene having a density of 0.930 g/cm³ or lower has a softening temperature lower than 100 ° C. so that it can perform the function as the sliding layer because its stiffness S(t) after one year at 100° C. is 10³ N/m² or lower.

For maleic acid modified polyethylene having a low density (0.930 g/cm³ or lower), the lower limit of density need not be specified because the condition that the stiffness S(t) after one year at 130° C. is 10⁻⁵ N/m² or higher is met. Maleic acid modified polyethylene has high adhesion properties to the steel of the oil well pipe body because it has a polar group of maleic acid.

Ethylene-vinyl acetate copolymer can provide a material for meeting the condition of the above range of stiffness S(t) in the intended temperature range by selecting the optimum composition. Because vinyl acetate has a lower softening temperature than ethylene component, the stiffness S(t) can freely be selected in a fixed range by changing the compounding proportion.

To meet the condition of the above range of stiffness S(t) in the temperature range of 40° to 130° C. the content of vinyl acetate in ethylene-vinyl acetate copolymer may be set at 5 to 60 percent by weight.

Also, maleic acid may be allowed to react with ethylene-vinyl acetate copolymer to yield maleic acid modified ethylene-vinylacetate copolymer in order to form the sliding layer. In this case, the adhesion properties to the steel of the oil well pipe body are high because maleic acid is a polar group.

For the same reason, a material in which maleic acid modified polyethylene is added to ethylene-vinyl acetate copolymer may be used as the sliding layer.

An additive etc. may be mixed with the aforementioned resins as necessary. Such a sliding layer can be formed on the outer peripheral surface of the oil well pipe body, for example, by extrusion coating.

Next, the reason for providing a protective layer will be described. The protective layer serves to prevent the damage to the sliding layer and the oil well pipe body when the oil well pipe is transported or buried. The present invention includes the case where the oil well pipe has a protective layer as necessary.

For the protective layer, a material is used which does not soften in the environment in which the oil well pipe is used. When the upper limit of the working temperature is 130° C. a material having a softening point exceeding 130° C. may be used.

Such materials include polypropylene and polyolefin resin such as ethylene-propylene block copolymer. Preferably, polypropylene or ethylene-propylene block copolymer having a softening temperature of 140° to 170° C. should be used.

When the sliding layer formed from maleic acid modified polyethylene is coated with ethylene-propylene block copolymer, both materials are desirable ones because they have high adhesion properties. Like the sliding layer, an additive etc. can be mixed in the protective layer as necessary.

The protective layer formed from resin can be formed on the outer peripheral surface of the sliding layer, for example, by extrusion coating. On the other hand, a metal such as mild steel is useful as the protective layer. For example, mild steel band may be wound in a spiral form to provide the protective layer. Needless to say, the presence of protective layer has no influence on the frictional force acting on the oil well pipe.

EXAMPLE

An oil well pipe in accordance with a example of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a sectional view of the oil well pipe of this invention. In this figure, reference numeral 1 denotes an oil well pipe body, 2 denotes a sliding layer, and 3 denotes a protective layer.

FIG. 2 is a schematic view illustrating a method for manufacturing the oil well pipe. The oil well pipe is manufactured by applying extrusion coating of sliding layer 2 on the outer peripheral surface of the oil well pipe body 1 by means of a sliding layer extruding machine 4 and by applying extrusion coating of protective layer 3 on the outer peripheral surface of the sliding layer 2 by means of a protective layer extruding machine 5.

The steel pipe used is an oil well pipe having an outside diameter of 177.8 mm, a wall thickness of 23.8 mm, and a length of 12,000 mm. On its outer peripheral surface, the sliding layer 2 was formed by extruding melted asphalt, polyethylene, maleic acid modified polyethylene, ethylene-vinyl acetate copolymer, maleic acid modified vinylacetate copolymer, or a mixture of ethylene-vinyl acetate copolymer and maleic acid modified polyethylene by using the sliding layer extruding machine 4. The thickness of the sliding layer is 1 mm for Example Nos. 10, 13, and 15 and Control Nos. 1 and 2, and 2 mm for others.

The protective layer 3 was formed on the outer peripheral surface of the sliding layer 2 by extruding melted polyethylene, polypropylene, or ethylene-propylene block polymer by using the protective layer extruding machine 5. The thickness of the protective layer is 4 mm for Example No. 10, and 3 mm for others. Further, an oil well pipe having a protective layer in which a 0.3 mm-thick, 200 mm-wide mild steel band was wound in a spiral form was manufactured. Needless to say, pipes having no protective layer were manufactured as the examples and the controls. Among these examples and controls, Example Nos. 1 to 15 and Control Nos. 1 to 4 are given in Table 1, Example Nos. 1 to 4 in Table 2, and Example Nos. 45 to 60 in Table 3.

For the oil well pipes of Example Nos. 1 to 60 and Control Nos. 1 to 4, the stiffness S(t) of the sliding layer at temperatures of 40° C., 50° C., 60° C., 70° C., 80° C., 90° C., 100° C., 110° C., 120° C., and 130° C. after one year has elapsed, and the stability of the protective layer after being left in a furnace of the same temperatures for one hour were examined. The results of these examinations are given in Tables 4 to 6.

The oil well pipes of Example Nos. 1 to 60 can be used in the temperature range in which the stiffness S(t) after one year has elapsed is in the range of 10⁻⁵ to 10³ N/m². In this temperature range, there is little possibility that the oil well pipes are destructed by ground subsidence. It is very safe to use oil well pipes in the temperature range in which the stiffness S(t) is in the range of 10⁻⁵ to 10² N/m².

Contrarily, it is found that the stiffness S(t) after one year has elapsed exceeds 10³ N/m² at temperatures not higher than 120° C. in Control Nos. 1 and 2 and at any temperatures in Control Nos. 3 and 4, so that the frictional force is high, and the oil well pipes are prone to be destroyed by ground subsidence.

The oil well pipes in Example Nos. 1 to 60 show no abnormality in a wide temperature range, so that the damage to the sliding layer and oil well pipe body can be prevented not only when the oil well pipe is transported but also when it is buried.

Thus, the oil well pipe having the sliding layer formed from a material consisting mainly of asphalt, polyethylene, maleic acid modified polyethylene, ethylene-vinyl acetate copolymer, maleic acid modified vinylacetate copolymer, or a mixture of these materials is very safe against the destruction caused by ground subsidence when it is used in a temperature range of normal temperature to 130° C.

The present invention prevents the destruction of oil well pipes caused by ground subsidence, and also prevents the damage to the sliding layer and the oil well pipe body not only when the oil well pipe is transported but also when it is buried.

                                      TABLE 1                                      __________________________________________________________________________           Number of                Thick-               Thick-                           Example or               ness                                                                               Density                                                                             Material for Protective                                                                    ness                       Items Control                                                                              Material for Sliding Layer                                                                        (mm)                                                                               (g/cm.sup.3)                                                                        Layer       (mm)                       __________________________________________________________________________     Example of                                                                           1     Mixed Asphalt      2   --   None        --                         Present                                                                              2     Heated Asphalt     2   --   None        --                         Invention                                                                            3     Mixed Asphalt      2   --   Polyethylene                                                                               3                                4     Heated Asphalt     2   --   Polypropylene                                                                              3                                5     Heated Asphalt     2   --   Mild Steel    0.3                            6     Polyethylene       2   0.925                                                                               None        --                               7     Polyethylene       2   0.912                                                                               Polypropylene                                                                              3                                8     Polyethylene       2   0.900                                                                               Mild Steel    0.3                            9     Maleic Acid Modified Polyethylene                                                                 2   0.928                                                                               No layer    --                               10    Maleic Acid Modified Polyethylene                                                                 1   0.928                                                                               Polypropylene                                                                              4                                11    Maleic Acid Modified Polyethylene                                                                 2   0.928                                                                               Polypropylene                                                                              3                                12    Maleic Acid Modified Polyethylene                                                                 2   0.928                                                                               E.P.B.C.    3                                13    Maleic Acid Modified Polyethylene                                                                 1   0.915                                                                               E.P.B.C.    3                                14    Maleic Acid Modified Polyethylene                                                                 2   0.900                                                                               E.P.B.C.    3                                15    Maleic Acid Modified Polyethylene                                                                 1   0.900                                                                               Mild Steel    0.3                      Control                                                                              1     Maleic Acid Modified Polyethylene                                                                 1   0.936                                                                               None        --                               2     Maleic Acid Modified Polyethylene                                                                 1   0.936                                                                               Polyethylene                                                                               1                                3     Polypropylene      2   --   E.P.B.C.    3                                4     Ethylene-Propylene Block Copolymer                                                                2   --   Polypropylene                                                                              3                          __________________________________________________________________________      Note:                                                                          "E.P.B.C." means EthylenePropylene Block Copolymer.                      

                                      TABLE 2                                      __________________________________________________________________________           Number of               Thick-                                                                             Mixing  Mixing           Thick-                    Example or              ness                                                                               Proportion                                                                             Proportion                                                                            Material                                                                                 ness                Items Control                                                                              Material for Sliding Layer                                                                       (mm)                                                                               of A (wt. %)                                                                           of B (wt. %)                                                                          Protective                                                                               (mm)r               __________________________________________________________________________     Example of                                                                           16    Ethylene-Vinyl Acetate Copolymer                                                                 2    6       0     None      --                  Present                                                                              17    Ethylene-Vinyl Acetate Copolymer                                                                 2    6       0     E.P.B.C.  3                   Invention                                                                            18    Ethylene-Vinyl Acetate Copolymer                                                                 2    6       0     Polypropylene                                                                            3                         19    Ethylene-Vinyl Acetate Copolymer                                                                 2    6       0     Mild Steel                                                                                 0.3                     20    Ethylene-Vinyl Acetate Copolymer                                                                 2    6       0     Polyethylene                                                                             3                         21    Ethylene-Vinyl Acetate Copolymer                                                                 2   10       0     Polypropylene                                                                            3                         22    Ethylene-Vinyl Acetate Copolymer                                                                 2   14       0     E.P.B.C.  3                         23    Ethylene-Vinyl Acetate Copolymer                                                                 2   19       0     E.P.B.C.  3                         24    Ethylene-Vinyl Acetate Copolymer                                                                 2   19      10     E.P.B.C.  3                         25    Ethylene-Vinyl Acetate Copolymer                                                                 2   19      25     None      --                        26    Ethylene-Vinyl Acetate Copolymer                                                                 2   19      25     E.P.B.C.  3                         27    Ethylene-Vinyl Acetate Copolymer                                                                 2   19      25     Polypropylene                                                                            3                         28    Ethylene-Vinyl Acetate Copolymer                                                                 2   19      25     Mild Steel                                                                                 0.3                     29    Ethylene-Vinyl Acetate Copolymer                                                                 2   19      25     Polyethylene                                                                             3                         30    Ethylene-Vinyl Acetate Copolymer                                                                 2   19      50     Polyethylene                                                                             3                         31    Ethylene-Vinyl Acetate Copolymer                                                                 2   25       0     E.P.B.C.  3                         32    Ethylene-Vinyl Acetate Copolymer                                                                 2   33       0     Polyethylene                                                                             3                         33    Ethylene-Vinyl Acetate Copolymer                                                                 2   33      10     E.P.B.C.  3                         34    Ethylene-Vinyl Acetate Copolymer                                                                 2   33      25     Polyethylene                                                                             3                         35    Ethylene-Vinyl Acetate Copolymer                                                                 2   33      50     E.P.B.C.  3 3                       36    Ethylene-Vinyl Acetate Copolymer                                                                 2   41       0     E.P.B.C.  3                         37    Ethylene-Vinyl Acetate Copolymer                                                                 2   46       0     Polyethylene                                                                             3                         38    Ethylene-Vinyl Acetate Copolymer                                                                 2   46      10     E.P.B.C.  3                         39    Ethylene-Vinyl Acetate Copolymer                                                                 2   46      25     Polyhylene                                                                               3                         40    Ethylene-Vinyl Acetate Copolymer                                                                 2   46      50     None      --                        41    Ethylene-Vinyl Acetate Copolymer                                                                 2   46      50     E.P.B.C.  3                         42    Ethylene-Vinyl Acetate Copolymer                                                                 2   46      50     Polypropylene                                                                            3                         43    Ethylene-Vinyl Acetate Copolymer                                                                 2   46      50     Mild Steel                                                                                 0.3                     44    Ethylene-Vinyl Acetate Copolymer                                                                 2   46      50     Polyethylene                                                                             3                   __________________________________________________________________________      Notes                                                                          1. "A" shown in the item column means "VinyleAcetate in                        EthyleneVinylacetate Copolymer".                                               2. "B" shown in the item column means "Maleic Acid Modified Polyethylene"      3. "E.P.B.C." means EthylenePropylene Block Copolymer.                   

                                      TABLE 3                                      __________________________________________________________________________            Number of                     Thick-                                                                             Mixing           Thick-                      Example or                    ness                                                                               Proportion                                                                             Material                                                                                ness                 Items  Control                                                                              Material for Sliding Layer                                                                             (mm)                                                                               of A (wt. %)                                                                           Protective                                                                              (mm)r                __________________________________________________________________________     Example of                                                                            45    Maleic Acid Modified Vinylacetate Copolymer                                                            2    6      None     --                   Present                                                                               46    Maleic Acid Modified Vinylacetate Copolymer                                                            2    6      E.P.B.C. 3                    Invention                                                                             47    Maleic Acid Modified Vinylacetate Copolymer                                                            2    6      Polypropylene                                                                           3                           48    Maleic Acid Modified Vinylacetate Copolymer                                                            2    6      Mild Steel                                                                                0.3                       49    Maleic Acid Modified Vinylacetate Copolymer                                                            2    6      Polyethylene                                                                            3                           50    Maleic Acid Modified Vinnylacetate Copolymer                                                           2   10      Polypropylene                                                                           3                           51    Maleic Acid Modified Vinylacetate Copolymer                                                            2   14      Polypropylene                                                                           3                           52    Maleic Acid Modified Vinylacetate Copolymer                                                            2   19      Polypropylene                                                                           3                           53    Maleic Acid Modified Vinylacetate Copolymer                                                            2   25      Polpropylene                                                                            3                           54    Maleic Acid Modified Vinylacetate Copolymer                                                            2   33      Polypropylene                                                                           3                           55    Maleic Acid Modified Vinylacetate Copolymer                                                            2   41      Polypropylene                                                                           3                           56    Maleic Acid Modified Vinylacetate Copolymer                                                            2   46      None     --                          57    Maleic Acid Modified Vinylacetate Copolymer                                                            2   46      E.P.B.C. 3                           58    Maleic Acid Modified Vinylacetate Copolymer                                                            2   46      Polypropylene                                                                           3                           59    Maleic Acid Modified Vinylacetate Copolymer                                                            2   46      None     --                          60    Maleic Acid Modified Vinylacetate Copolymer                                                            2   46      Polyethylene                                                                            3                    __________________________________________________________________________      Notes                                                                          1. "A" shown in the item column means "VinylAcetate in                         EthyleneVinylacetate Copolymer".                                               2. "E.P.B.C." means EthylenePropylene Block Copolymer.                   

                                      TABLE 4                                      __________________________________________________________________________                                                              Prefer-               Number                                                   able                  of                                                       Temper-               Example                                           Usable ature                 or        Stiffness S(t) of Sliding Layer (N/m.sup.2)                                                                            Temperature                                                                           Range                 Items                                                                               Control                                                                             40° C.                                                                      50° C.                                                                      60° C.                                                                      70° C.                                                                      80° C.                                                                      90° C.                                                                      100° C.                                                                     110° C.                                                                     120° C.                                                                     130° C.                                                                     Range                                                                                 (°C.))         __________________________________________________________________________     Example                                                                             1    10.sup.3                                                                           10.sup.1                                                                           10.sup.-3                                                                          10.sup.-5                                                                          <10.sup.-5              40˜70                                                                           50˜70           of   2    10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5  50˜100                                                                          50˜100          Present                                                                             3    10.sup.3                                                                           10.sup.1                                                                           10.sup.-3                                                                          10.sup.-5                                                                          <10.sup.-5              40˜70                                                                           50˜70           Invention                                                                           4    10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5  50˜100                                                                          50˜100               5    10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5  50˜100                                                                          50˜100               6            >10.sup.7                                                                          10.sup.7                                                                           10.sup.6                                                                           10.sup.4                                                                           10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          100˜130                                                                         100˜130              7            >10.sup.7                                                                          10.sup.4                                                                           10.sup.5                                                                           10.sup.2                                                                           10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-2                                                                          90˜130                                                                          90˜130               8            >10.sup.7                                                                          10.sup.4                                                                           10.sup.5                                                                           10.sup.2                                                                           10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-3                                                                          90˜130                                                                          90˜130               9            >10.sup.7                                                                          10.sup.7                                                                           10.sup.6                                                                           10.sup.3                                                                           10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          90˜130                                                                          100˜130              10           >10.sup.7                                                                          10.sup.7                                                                           10.sup.6                                                                           10.sup.3                                                                           10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          90˜130                                                                          100˜130              11           >10.sup.7                                                                          10.sup.7                                                                           10.sup.6                                                                           10.sup.3                                                                           10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          90˜130                                                                          100˜130              12           >10.sup.7                                                                          10.sup.7                                                                           10.sup.6                                                                           10.sup.3                                                                           10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          90˜130                                                                          100˜130              13           >10.sup.7                                                                          10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          90˜130                                                                          90˜130               14           >10.sup.7                                                                          10.sup.6                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          90˜130                                                                          90˜130               15           >10.sup.7                                                                          10.sup.6                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          90˜130                                                                          90˜130          Control                                                                             1                    >10.sup.7                                                                          10.sup.7                                                                           2 ×                                                                          10.sup.6                                                                           10.sup.5                                                                           10.sup.-2                                                                          130    130                                                     10.sup.6                                          2                    >10.sup.7                                                                          10.sup.7                                                                           2 ×                                                                          10.sup.6                                                                           10.sup.5                                                                           10.sup.-2                                                                          130    130                                                     10.sup.6                                          3                                >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           None   None                       4                                >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           None   None                  __________________________________________________________________________

                                      TABLE 5 (A)                                  __________________________________________________________________________                                                              Prefer-                                                                        able                                                                           Temper-               Number                                            Usable ature                 of        Stiffness S(t) of Sliding Layer (N/m.sup.2)                                                                            Temperature                                                                           Range                 Items                                                                               Example                                                                             40° C.                                                                      50° C.                                                                      60° C.                                                                      70° C.                                                                      80° C.                                                                      90° C.                                                                      100° C.                                                                     110° C.                                                                     120° C.                                                                     130° C.                                                                     Range                                                                                 (°C.))         __________________________________________________________________________     Example                                                                             16   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130         of   17   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130         Present                                                                             18   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130         Invention                                                                           19   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130              20   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130              21   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.3                                                                           10.sup.2                                                                           10.sup.-1                                                                          10.sup.-2                                                                          100˜130                                                                         110˜130              22   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.3                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          90˜130                                                                          100˜130              23   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               31   10.sup.6                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.3                                                                           10.sup.0                                                                           10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         80˜120                                                                          90˜120               32   10.sup.5                                                                           10.sup.3                                                                           10.sup.1                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         60˜120                                                                          50˜120               36   10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-3                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         50˜120                                                                          50˜120               37   10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         40˜110                                                                          40˜120               24   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               25   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               26   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               27   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               28   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               29   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               30   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               33   10.sup.5                                                                           10.sup.4                                                                           10.sup.3                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         60˜120                                                                          70˜120               34   10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          <10.sup.-5                                                                         70˜120                                                                          70˜120               35   10.sup.6                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.2                                                                           10.sup.-1                                                                          10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          <10.sup.-5                                                                         70˜120                                                                          70˜120          __________________________________________________________________________

                                      TABLE 5 (B)                                  __________________________________________________________________________                                                              Prefer-                                                                        able                                                                           Temper-               Number                                            Usable ature                 of        Stiffness S(t) of Sliding Layer (N/m.sup.2)                                                                            Temperature                                                                           Range                 Items                                                                               Example                                                                             40° C.                                                                      50° C.                                                                      60° C.                                                                      70° C.                                                                      80° C.                                                                      90° C.                                                                      100° C.                                                                     110° C.                                                                     120° C.                                                                     130° C.                                                                     Range                                                                                 (°C.))         __________________________________________________________________________          38   10.sup.3                                                                           10.sup.3                                                                           10.sup.3                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         40˜110                                                                          70˜110               39   10.sup.4                                                                           10.sup.4                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         70˜110                                                                          70˜110               40   10.sup.5                                                                           10.sup.5                                                                           10.sup.5                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         70˜110                                                                          70˜110               41   10.sup.5                                                                           10.sup.5                                                                           10.sup.5                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         70˜110                                                                          70˜110               42   10.sup.5                                                                           10.sup.5                                                                           10.sup.5                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         70˜110                                                                          70˜110               43   10.sup.5                                                                           10.sup.5                                                                           10.sup.5                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         70˜110                                                                          70˜110               44   10.sup.5                                                                           10.sup.5                                                                           10.sup.5                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         70˜110                                                                          70˜110          Example                                                                             45   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130         of   46   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130         Present                                                                             47   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130         Invention                                                                           49   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-2                                                                          110˜130                                                                         110˜130              50   >10.sup.7                                                                          10.sup.7                                                                           10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.3                                                                           10.sup.2                                                                           10.sup.-1                                                                          10.sup.-2                                                                          100˜130                                                                         110˜130              51   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.3                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          90˜130                                                                          100˜130              52   10.sup.7                                                                           10.sup.7                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.1                                                                           10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          90˜130                                                                          90˜130               53   10.sup.6                                                                           10.sup.6                                                                           10.sup.5                                                                           10.sup.4                                                                           10.sup.3                                                                           10.sup.0                                                                           10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         80˜120                                                                          90˜120               54   10.sup.5                                                                           10.sup.3                                                                           10.sup.1                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         60˜120                                                                          50˜120               55   10.sup.4                                                                           10.sup.2                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-3                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         50˜120                                                                          50˜120               56   10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         40˜110                                                                          40˜110               57   10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         40˜110                                                                          40˜110               58   10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         40˜110                                                                          40˜110               59   10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         40˜110                                                                          40˜110               60   10.sup.0                                                                           10.sup.0                                                                           10.sup.-1                                                                          10.sup.-2                                                                          10.sup.-3                                                                          10.sup.-3                                                                          10.sup.-4                                                                          10.sup.-5                                                                          <10.sup.-5                                                                         <10.sup.-5                                                                         40˜110                                                                          40˜110          __________________________________________________________________________

                  TABLE 6(C)                                                       ______________________________________                                                  No. of                                                                         Example    Durability of                                                       or         Protective Layer                                           Items      Control      <120° C.                                                                         130° C.                                ______________________________________                                         Example of 45           --       --                                            Present    46           ∘                                                                           ∘                                 Invention  47           ∘                                                                           ∘                                            48           ∘                                                                           ∘                                            49           ∘                                                                           x                                                        50           ∘                                                                           ∘                                            51           ∘                                                                           ∘                                            52           ∘                                                                           ∘                                            53           ∘                                                                           ∘                                            54           ∘                                                                           ∘                                            55           ∘                                                                           ∘                                            56           --       --                                                       57           ∘                                                                           ∘                                            58           ∘                                                                           ∘                                            59           ∘                                                                           ∘                                            60           ∘                                                                           x                                             ______________________________________                                          Notes:                                                                         Symbol of "∘" means "durable"; and Symbol of "x" softened and      deformed".                                                               

                  TABLE 6(B)                                                       ______________________________________                                                  No. of                                                                         Example    Durability of                                                       or         Protective Layer                                           Items      Control      <120° C.                                                                         130° C.                                ______________________________________                                         Example of 16           --       --                                            Present    17           ∘                                                                           ∘                                 Invention  18           ∘                                                                           ∘                                            19           ∘                                                                           ∘                                            20           ∘                                                                           x                                                        21           ∘                                                                           ∘                                            22           ∘                                                                           ∘                                            23           ∘                                                                           ∘                                            24           ∘                                                                           ∘                                            25           --       --                                                       26           ∘                                                                           ∘                                            27           ∘                                                                           ∘                                            28           ∘                                                                           ∘                                            29           ∘                                                                           x                                                        30           ∘                                                                           x                                                        31           ∘                                                                           ∘                                            32           ∘                                                                           x                                                        33           ∘                                                                           ∘                                            34           ∘                                                                           x                                                        35           ∘                                                                           ∘                                            36           ∘                                                                           ∘                                            37           ∘                                                                           x                                                        38           ∘                                                                           ∘                                            39           ∘                                                                           x                                                        40           --       --                                                       41           ∘                                                                           ∘                                            42           ∘                                                                           ∘                                            43           ∘                                                                           ∘                                            44           ∘                                                                           x                                             ______________________________________                                          Notes:                                                                         Symbol of "∘" means "durable"; and Symbol of "x" softened and      deformed".                                                               

                  TABLE 6(A)                                                       ______________________________________                                                  No. of                                                                         Example    Durability of                                                       or         Protective Layer                                           Items      Control      <120° C.                                                                         130° C.                                ______________________________________                                         Example of 1            --       --                                            Present    2            --       --                                            Invention  3            ∘                                                                           x                                                        4            ∘                                                                           ∘                                            5            ∘                                                                           ∘                                            6            --       --                                                       7            ∘                                                                           ∘                                            8            ∘                                                                           ∘                                            9            --       --                                                       10           ∘                                                                           ∘                                            11           ∘                                                                           ∘                                            12           ∘                                                                           ∘                                            13           ∘                                                                           ∘                                            14           ∘                                                                           ∘                                            15           ∘                                                                           ∘                                 Control    1            --       --                                                       2            ∘                                                                           x                                                        3            ∘                                                                           ∘                                            4            ∘                                                                           ∘                                 ______________________________________                                          Notes:                                                                         Symbol of "∘" means "durable"; and Symbol of "x" softened and      deformed".                                                                

What is claimed is:
 1. An oil well pipe for a casing comprising:an oil well pipe body; a sliding layer formed on an outside surface of said oil well pipe body, said sliding layer comprising an ethylene-vinyl acetate copolymer, said ethylene-vinyl acetate copolymer containing vinyl acetate in an amount of 5 to 60 percent by weight; and a protective layer which is formed on an outside surface of the sliding layer.
 2. The oil well pipe of claim 1, wherein said protective layer comprises polyolefin resin.
 3. The oil well pipe of claim 2, wherein the polyolefin resin is an ethylene-propylene block copolymer.
 4. The oil well pipe of claim 2, wherein the ethylene-propylene block copolymer has a softening temperature of 140° to 170° C.
 5. The oil well pipe of claim 4, wherein the sliding layer has a thickness of 1 mm to 2 mm and the protective layer has a thickness of 3 mm to 4 mm.
 6. An oil well pipe for a casing comprising:an oil well pipe body; a sliding layer formed on an outside surface of said oil well pipe body, said sliding layer comprising an ethylene-vinyl acetate copolymer and maleic acid modified polyethylene, said ethylene-vinyl acetate copolymer containing vinyl acetate in an amount of 5 to 60 percent by weight; and a protective layer which is formed on an outside surface of the sliding layer.
 7. The oil well pipe of claim 6, wherein said protective layer comprises polyolefin resin.
 8. The oil well pipe of claim 7, wherein the polyolefin resin is an ethylene-propylene block copolymer.
 9. The oil well pipe of claim 8, wherein the ethylene-propylene block copolymer has a softening temperature of 140° to 170° C.
 10. The oil well pipe of claim 9, wherein the sliding layer has a thickness of 1 mm to 2 mm and the protective layer has a thickness of 3 mm to 4 mm.
 11. An oil well pipe for a casing comprising:an oil well pipe body; a sliding layer formed on an outside surface of said oil well pipe body, said sliding layer comprising a maleic acid modified ethylene-vinyl acetate copolymer, said maleic acid modified ethylene-vinyl acetate copolymer containing vinyl acetate in an amount of 5 to 60 percent by weight; and a protective layer which is formed on an outside surface of the sliding layer.
 12. The oil well pipe of claim 11, wherein said protective layer comprises polyolefin resin.
 13. The oil well pipe of claim 12, wherein the polyolefin resin is an ethylene propylene block copolymer.
 14. The oil well pipe of claim 13, wherein the ethylene-propylene block copolymer has a softening temperature of 140° to 170° C.
 15. The oil well pipe of claim 14, wherein the sliding layer has a thickness of 1 mm to 2 mm and the protective layer has a thickness of 3 mm to 4 mm. 